Homework Help: Px = P cos θx equation question.

1. Dec 1, 2011

AcousticBruce

$$P_x = P cos θ_x$$

is Px always the adjacent line of the triangle?

2. Dec 1, 2011

RTW69

Yes, if the adjacent side is of a right triangle, where P is the hypotenuse and θ is the angle between the Hypotenuse and adjacent side.

3. Dec 1, 2011

AcousticBruce

Thank you. And a new question.

Why would it be Px for adjacent and on the other side be P without a subscript?

4. Dec 1, 2011

Staff: Mentor

Presumably a vector of magnitude P is being decomposed into its X and Y components, with θx being the angle between P and the X axis. Thus:

Px = P cos(θx)

and

Py = P sin(θx)

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Last edited: Dec 1, 2011
5. Dec 1, 2011

RTW69

Your problem tells me that Px is the component of P in the x-direction. The component of P in the y-direction would be labeled Py. P is the hypotenuse. P might be a force at some angle θ from the x-axis and you are trying to find the component of that force in the x-direction which utilizes the Cosine trig function. If you were trying to find component of the force in the y-direction, Py you would use the sine trig function.

6. Dec 1, 2011

AcousticBruce

So would you ever really have the θy?

7. Dec 1, 2011

RTW69

Yes, look at gneil's image. θx is measured from the x-axis. θy would be the angle measured from the y-axis and would equal 90 degrees- θx.

8. Dec 1, 2011

Staff: Mentor

Sure. You never know which angle may be given to you or which one might be determined by some other factor in the problem. Example: problems that specify angles for pendulum strings which reference the vertical direction, not the horizontal.

9. Dec 1, 2011

AcousticBruce

Thanks a lot guys. Really! I am in a self study of physics and trigonometry. I am studying electronics and it seems that physics and trig would be a good thing to learn. :)